1. Field of the Invention
The invention relates to high intensity discharge lamps having tungsten or thoriated tungsten main electrodes; and more particularly, to discharge lamps whose electrodes include a crevice-forming coil of tungsten wire.
As interest in development of light sources with improved color characteristics has grown, and the use of many different metal halide additives to the lamp filling material has been investigated, a variety of problems affecting lamp life and lamp-to-lamp uniformity have arisen. Various attempts at improving this situation have been suggested over the years, and an improved high color rendering metal halide lamp and method of manufacturing it is disclosed in the concurrently filed application by applicant and Michael H. Masto, hereby incorporated by reference.
2. Description of the Prior Art
Electrodes for metal halide high intensity discharge lamp are well known, and commonly comprise a thoriated tungsten core extending between a molybdenum strip in the press seal of the arc tube and a tip from which the arc is desired to burn during operation, and a coil of a few turns of tungsten wire, generally of a same or smaller diameter than the core, wound tightly about the core a short distance from the tip. When an arc is first struck within the lamp, the arc usually initiates at one of the crevices formed by the coil of wire; good operation and life of the bulb requires that preferably within five seconds, and certainly within fifteen seconds, that the arc spot transfer to the tip of the electrode. However, inefficient transfer of the hot spot from the coil to the thoriated tip has long been known as a source of instability in metal halide discharge lamps.
Another problem observed in the prior art is the blackening of discharge envelope wall, apparently due to the deposition of tungsten which has been evaporated from the electrodes; this effect has been described as particularly marked when scandium iodide is one of the additives. U.S. Pat. No. 4,232,243 describes this effect, and one solution to it by which each electrode is formed with a cavity, and the cavity is filled with a glassy or solidified melt substantially filling the cavity and adhering to the electrode structure, this solidified melt being an activator material consisting essentially of scandium oxide. As described in that patent, the presence of the scandium oxide facilitates rapid starting of the discharge, and provides a luminous efficacy equal to that obtained with unactivated tungsten electrodes. To reduce blackening of the envelope, this patent teaches the addition of a metal bromide to the additives in the lamp, and the use of a tungsten wire electrode instead of the more common thoriated tungsten.
Finally, metal halide lamps are characterized by reignition voltage spikes that develop after the lamp has warmed up. To facilitate starting and avoid excessive reignition voltage, the electrode coil should not compete with the electrode tip for arc formation. If the electrode design is such that the arc forms at the electrode coil as mentioned above, then the arc should transfer rapidly to the electrode tip and should extinguish at every other portion of the electrode.